1. Field of the Invention
The present invention relates to a roller type hemming apparatus for hemming, for example, of a door panel for a vehicle.
2. Description of the Prior Art
Conventionally, a door panel has three edges which are formed by hemming layers of an outer panel and an inner panel. An example of a prior art apparatus to be used for such hemming is described in Japanese Laid-Open Patent Publication No. 61-262432 in which a roller type hemming apparatus is disclosed by the applicant of the present application.
The conventional hemming apparatus is illustrated generally at h in FIG. 11. As shown therein, the hemming apparatus h includes shifting means r such as a multi-axis controllable robot hand having a predetermined traveling path and a hem roller m carried by the shifting means r, and is adapted for hemming a bent portion We of a workpiece W which is positioned on a lower die k by the hem roller m rotated under pressure along the bent portion We. As illustrated, the bent portion We is bent in advance substantially at right angles, and the hem roller m is rotated under pressure along the extreme end of the bent portion We to achieve preliminary bending to a predetermined angle. Thereafter, the rolling operation is repeated to completely fold over the bent portion We, as shown in FIG. 12.
The roller type hemming apparatus h can be used for continuous hemming along the contour of the peripheral edge of a workpiece W, thus assuring good product accuracy as well as improved operating efficiency. The roller type hemming apparatus h as described above, however, involves the following problems.
Specifically, as shown in FIG. 12, hemming is achieved by shifting the hem roller m along the bent portion We in a pressed condition, and in order to achieve good hemming by folding over the bent portion We completely, the hem roller m must be shifted with its lower end always held at a constant distance .delta. away from a forming surface (or upper surface) ko of the lower die k. Therefore, an ideal traveling path (hereinafter referred to as "hemming level LH") must be defined for the hem roller shifting means r such as a robot hand. Such a constant distance .delta. is equal to the total of twice the thickness of an outer panel Wo and the thickness of an inner panel Wi.
The hem roller shifting means r is not, however, composed of a perfect rigid body, and when the hem roller m is pressed against the bent portion We for hemming, the shifting means r may be deflected through resiliency of itself and repulsive force of the bent portion We. Therefore, as shown in FIG. 13, the actual traveling path (hereinafter referred to as "traveling level LI") of the hem roller m is deviated by a distance B from the hemming level LH. This results in insufficient pressing force applied by the hem roller m and therefore imperfect folding over of the bent portion We, which will adversely affect satisfactory hemming. Thus, there has been a problem that the hemming level LH does not coincide with the actual traveling level LI due to the deflection of the hem roller shifting means r.
Furthermore, as the bent portion We of the workpiece W such as a door panel to be hemmed usually includes complex and continuous curves or the like, it has been quite difficult and sometimes practically impossible to define such a traveling path for the hem roller shifting means r as to permit accurate follow-up of the hem roller m along the contour of the bent portion We. Therefore, the traveling path of the hem roller m during the actual hemming operation includes slight difference from the contour of the bent portion We, which causes another difficulty in achieving good hemming.
When a multi-axis controllable robot hand as described above is used for the hem roller shifting means r, it is required to teach a predetermined traveling path to the robot hand. It is, however, impossible to directly and accurately teach such an ideal traveling path of the hem roller m as required, or the hemming level LH. This is because the hemming level LH is a spatial path held at a constant distance away from the forming surface ko of the lower die k. Thus, in case the robot hand is used for the hem roller shifting means r, this teaching procedure has been quite difficult.